The DNR says officials will not declare the fire is completely out until its source has been found. A three-person crew entered the Soudan Mine Sunday, but did not find the source. The workers restored electricity and turned on pumps that remove water that seeps from the ground above.

The fire was discovered about 9 p.m. Thursday when smoke alarms were activated.

The Soudan Underground Mine State Park is a popular tourist attraction in northeastern Minnesota between May and October. It’s also the location of a University of Minnesota physics lab, which officials say was not damaged in the fire.